Loading Rate and Temperature Dependences of Flexural Fatigue Behavior of Pitch Based Carbon Fiber Unidirectional FRP Laminates

The flexural fatigue behavior of unidirectional CFRP laminates using two kinds of pitch based carbon fibers was investigated by three point bendind fatigue test at several levels of temperature and two levels of loading rate. By comparing the flexural fatigue behavior of the two types of CFRP laminates, the characteristics of pitch based CFRPs was clarified as follows: (1) The flexural fatigue behavior of these CFRPs changed considerably with loading rate, and temperature, as well as the number of loading cycles to failure. The dependence on loading rate, temperature or number of cycles to fatilure is clearly divided into two regions by the fracture mechanism of the CFRPs. The two fracture mechanisms are the tensile fracture on the tension side of specimen and the compressive fracture on the compression side of specimen. (2) The flexural fatigue strengths of these CFRPs considerably decrease with loading rate and temperature as well as number of loading cycles to failure in the region of compressive fracture. The fatigue strength does not change with loading rate, temperature or number of loading cycles to failure in the region of tensile fracture unlike the compressive fracture region. (3) The reciprocation law between time and temperature holds for the flexural fatigue strength as well as the flexural static strength. (4) In the region of tensile fracture, the fatigue strengths of the two types of CFRP differ, that is, the fatigue strength of CFRP, which includes the high tensile strength fiber, has a higher tensile fatigue strength. On the other hand, in the region of compressive fracture, the fatigue strengths of the two types of CFRP show the same value at the same temperature and loading rate.